23.7 Transition Metals

Many of the most important metals of modern society are transition metals. Transition metals, which occupy the d block of the periodic table (Figure 23.21), include such familiar elements as chromium, iron, nickel, and copper. They also include less familiar elements that have come to play important roles in modern technology, such as those in the high-performance jet engine pictured in Figure 23.1. In this section we consider some of the physical and chemical properties of transition metals.

Figure 23.21 Transition metals are those elements that occupy the d block of the periodic table.

Physical Properties

Some of the physical properties of the elements of the first transition series are listed in Table 23.5. Some of these properties, such as ionization energy and atomic radius, are characteristic of isolated atoms of the elements. Others, including density and melting point, are characteristic of the bulk solid metal.

The properties of individual atoms show very similar variations across each series. For example, notice that the bonding atomic radii of the transition metals shown in Figure 23.22 exhibit the same pattern of variation in the three series. The trend in atomic radii is complex because it is the product of several factors, some of which work in opposite directions. In general, we would expect the atomic radius to decrease more or less steadily as we proceed from left to right across the transition series, because of increasing effective nuclear charge. Indeed, for groups 3, 4, and 5, this is the trend observed. However, as the number of d electrons increases, not all of them are typically employed in bonding. Non-bonding electrons exert repulsive effects that cause increased bond distances, and we see their effects in the maximum that occurs at group 7, and in the increase seen as we move past the group 8 elements. The bonding atomic radius is an empirical number that is especially difficult to define for elements such as the transition metals, which can exist in various oxidation states. Nevertheless, comparisons of the variations from one series to another are valid.

Figure 23.22 Variation in atomic radius of transition metals as a function of the periodic table group number.

The incomplete screening of the nuclear charge by added electrons produces an interesting and important effect in the third transition-metal series. In general, the bonding atomic radius increases as we move down in a family because of the increasing principal quantum number of the outer shell electrons. Notice, however, that once we move beyond the group 3 elements, the second and third transition-series elements have virtually the same bonding atomic radii. For example, in group 5, tantalum has virtually the same radius as niobium. This effect has its origin in the lanthanide series, the elements with atomic numbers 58 through 71, which occur between La and Hf (Figure 23.21). The filling of 4f orbitals through the lanthanide elements causes a steady increase in the effective nuclear charge, producing a contraction in size, called the lanthanide contraction. This contraction just offsets the increase we would expect as we go from the second to the third series. Thus, the second- and third-series transition metals in each group have about the same radii all the way across the series. As a consequence, the second- and third-series metals in a given group have great similarity in their chemical properties. For example, the chemical properties of zirconium and hafnium are remarkably similar. They always occur together in nature, and they are very difficult to separate.

Electron Configurations and Oxidation States

Transition metals owe their location in the periodic table to the filling of the d subshells. When these metals are oxidized, however, they lose their outer s electrons before they lose electrons from the d subshell. For example, the electron configuration of Fe is [Ar]3d64s2, whereas that of Fe2+ is [Ar]3d6. Formation of Fe3+ requires loss of one 3d electron, giving [Ar]3d5. Most transition-metal ions contain partially occupied d subshells. The existence of these d electrons is partially responsible for several characteristics of transition metals:

They often exhibit more than one stable oxidation state.

Many of their compounds are colored (see Figure 23.23). We will discuss the origin of these colors in Chapter 24.

Figure 23.24 summarizes the common nonzero oxidation states for the first transition series. The oxidation states shown as large circles are those most frequently encountered either in solution or in solid compounds. The ones shown as small circles are less common. Notice that Sc occurs only in the +3 oxidation state and Zn occurs only in the +2 oxidation state. The other metals, however, exhibit a variety of oxidation states. For example, Mn is commonly found in solution in the +2 (Mn2+) and +7 (MnO4–) oxidation states. In the solid state the +4 oxidation state (as in MnO2) is common. The +3, +5, and +6 oxidation states are less common.

Figure 23.24 Nonzero oxidation states of the first transition series. The most common oxidation states are indicated by the larger circles.

The +2 oxidation state, which commonly occurs for nearly all of these metals, is due to the loss of their two outer 4s electrons. This oxidation state is commonly found for all these elements except Sc, where the 3+ ion with an [Ar] configuration is particularly stable.

Oxidation states above +2 are due to successive losses of 3d electrons. From Sc through Mn the maximum oxidation state increases from +3 to +7, equaling in each case the total number of 4s plus 3d electrons in the atom. Thus, manganese has a maximum oxidation state of 2 + 5 = +7. As we move to the right beyond the Mn in the first transition series, the maximum oxidation state decreases. In the second and third transition series the maximum oxidation state is +8, which is achieved in RuO4 and OsO4. In general, the maximum oxidation states are found only when the metals are combined with the most electronegative elements, especially O, F, and possibly Cl.

Magnetism

The magnetic properties of transition metals and their compounds are both interesting and important. Measurements of magnetic properties provide information about chemical bonding. In addition, many important uses are made of magnetic properties in modern technology.

An electron possesses a "spin" that gives it a magnetic moment; that is, it behaves like a tiny magnet. When a diamagnetic substance is placed in a magnetic field, the motions of the electrons are affected in such a way that the substance is very weakly repelled by the magnet. Figure 23.25(a) represents a diamagnetic solid, one in which all the electrons in the solid are paired.

Figure 23.25 Types of magnetic behavior. (a) Diamagnetic; no centers (atoms or ions) with magnetic moments. (b) Simple paramagnetic; centers with magnetic moments are not aligned unless the substance is in a magnetic field. (c) Ferromagnetic; coupled centers aligned in a common direction.

When an atom or ion possesses one or more unpaired electrons, the substance is paramagnetic. In a paramagnetic solid the unpaired electrons on the atoms or ions of the solid are not influenced by the electrons on adjacent atoms or ions. The magnetic moments on the individual atoms or ions are randomly oriented, as shown in Figure 23.25(b). When placed in a magnetic field, however, the magnetic moments become aligned roughly parallel to one another, producing a net attractive interaction with the magnet. Thus, a paramagnetic substance is drawn into a magnetic field.

You are probably much more familiar with the magnetic behavior of simple iron magnets (Figure 23.26), a much stronger form of magnetism called ferromagnetism. Ferromagnetism arises when the unpaired electrons of the atoms or ions in a solid are influenced by the orientations of the electrons of their neighbors. The most stable (lowest-energy) arrangement results when the spins of electrons on adjacent atoms or ions are aligned in the same direction, as shown in Figure 23.25(c). When a ferromagnetic solid is placed in a magnetic field, the electrons tend to align strongly along the magnetic field. The attraction for the magnetic field that results may be as much as 1 million times stronger than that for a simple paramagnetic substance. When the external magnetic field is removed, the interactions between the electrons cause the solid as a whole to maintain a magnetic moment. We then refer to it as a permanent magnet. The most common examples of ferromagnetic solids are the elements Fe, Co, and Ni. Many alloys exhibit greater ferromagnetism than do the pure metals themselves. Some metal oxides (for example, CrO2 and Fe3O4) are also ferromagnetic. Several ferromagnetic oxides are used in magnetic recording tape and computer disks.